Methods for preparing liquid-solid phase change proppant with a controllable particle size based on emulsified resin

ABSTRACT

Some embodiments of the present disclosure provide a method for preparing a self-phase change proppant based on an emulsified and toughened bio-based epoxy resin. Toughening modification is performed on the bio-based epoxy resin by graphite particles, and then the bio-based epoxy resin after the toughening modification is emulsified by SiO 2  particles as an emulsifier to prepare the self-phase change proppant; a proportion of different mesh numbers in the self-phase change proppant is adjusted by changing a concentration of the emulsifier during emulsification; and the chemical formula of the bio-based epoxy resin is: 
                         
The proppant particles in the present disclosure have good sphericity and high fracture permeability after being laid, which can effectively extract the remaining oil in the fractures, thus improving the development efficiency of the oilfield.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority of Chinese Patent Application No.202210159197.8, filed on Feb. 22, 2022, the entire contents of which areincorporated herein by reference.

TECHNICAL FIELD

The present disclosure generally relates to a field of the technology ofthe oilfield development, and in particular, to a method for preparing aself-phase change proppant based on an emulsified and toughenedbio-based epoxy resin.

BACKGROUND

Hydraulic fracturing technology has been widely used in major oilfieldsas the main technical means of reservoir renovation and enhancement ofoil recovery. In hydraulic fracturing technology, the injection of solidproppant to prevent the fractures from reclosing is the key to ensuringhigh conductivity of fractures. At present, the commonly used solidproppant in oilfields includes quartz sand, ceramsite, etc. Due to thehigh density of quartz sand, ceramsite, etc., highly viscous sand-ladenfluid usually needs to carry the solid proppant effectively, so thereare limitations in the use process. Secondly, to achieve the bestdevelopment effect, the current oilfield adopts the method of addingsand in different sections to prop up the fractures, that is, addingsmall-sized proppant first to prop up the distal fractures and thebranch fractures, then adding large-sized proppant to prop up the mainfractures and the near-well fractures, and the process of adding sandmultiple times leads to the increase of construction technology andconstruction cost.

Therefore, there is a need to provide a method for preparing aself-phase change proppant, which is used for preparing the self-phasechange proppant in low density that contains proppant particles ofdifferent particle sizes to meet the requirements of being laid indifferent sections.

SUMMARY

To solve the problems of the poor migration capacity of conventionalsolid proppant and complex process of being laid in different sectionsin the existing technology, some embodiments of the present disclosureprovide a method for preparing a self-phase change proppant based on anemulsified and toughened bio-based epoxy resin.

To solve the above technical problems, some embodiments of the presentdisclosure adopt the following technical solutions. A method forpreparing a self-phase change proppant based on an emulsified andtoughened bio-based epoxy resin, and toughening modification isperformed on the bio-based epoxy resin by graphite particles, and thenthe bio-based epoxy resin after the toughening modification isemulsified by SiO₂ particles as an emulsifier to prepare the self-phasechange proppant; a proportion of different mesh numbers in theself-phase change proppant is adjusted by changing a concentration ofthe emulsifier during emulsification; and the chemical formula of thebio-based epoxy resin is:

A method for preparing a self-phase change proppant based on anemulsified and toughened bio-based epoxy resin includes the followingsteps.

S1, preparing the bio-based epoxy resin.

S2, performing the toughening modification on the bio-based epoxy resinby using the graphite particles.

S3, using the SiO₂ particles as the emulsifier to emulsify the bio-basedepoxy resin after the toughening modification to prepare the self-phasechange proppant that is laid in sections.

In S2, a mass concentration of the graphite particles is 1%-7%.

In S2, a mass concentration of the graphite particles is 1%-5%.

In S2, a mass concentration of the graphite particles is 3%.

In S2, an average particle size of the graphite particles is 40 μm.

In S3, a particle size of the SiO₂ particles is 50 nm.

In S3, a mass concentration of the SiO₂ particles is 0.3-1%.

Bio-based epoxy resin is prepared as follows:

S101, using dimethyl sulfoxide as a solvent, placing eugenol and1-thioglycerol with a molar ratio of 1:1 and a catalyst4-dimethylaminopyridine (DMAP) with a total mass of 2% of the reactionmonomers in the solvent and heating them at 65° C. for 4 h to obtain afirst product.

S102, reacting the first product and epichlorohydrin in an ethanolsolution with a NaOH mass concentration of 10% at 85° C. for 6 h toobtain a second product, wherein the reaction formula is:

S103, using a saturated sodium bicarbonate solution to purify the secondproduct to obtain the target bio-based epoxy resin.

A viscosity and a density of the bio-based epoxy resin at 20° C. are4620 mPa·s and 1.2 g/cm³, respectively.

Some embodiments of the present disclosure have the following beneficialeffects over prior art.

(1) Some embodiments of the present disclosure provide the self-phasechange proppant based on an emulsified and toughened bio-based epoxyresin, which is easy to prepare, low in cost, stable in performance,capable of significantly reducing surface and interfacial tension, andhas a low concentration during use. Moreover, the self-phase changeproppant can effectively reduce the use of polymer surfactants whichused to reduce oil viscosity, and can effectively exploit the remainingoil in formation fractures, thereby improving the development efficiencyof the oilfield.

(2) The self-phase change proppant based on an emulsified and toughenedbio-based epoxy resin of some embodiments of the present disclosure hasgood migration ability and can meet the requirements of being laid indifferent sections. Due to the good sphericity of curing phase changeproppant particles, the fracture permeability of the curing phase changeproppant after being laid is better than that of quartz sand andceramsite proppants with the same particle size.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is further described in terms of exemplaryembodiments. These exemplary embodiments are described in detail withreference to the drawings. These embodiments are non-limiting exemplaryembodiments, in which like reference numerals represent similarstructures throughout the several views of the drawings, and wherein:

FIG. 1 illustrates the reaction formula of synthetizing a bio-basedepoxy resin according to some embodiments of the present disclosure;

FIG. 2 is a graph illustrating a product form of the bio-based epoxyresin according to some embodiments of the present disclosure;

FIG. 3 is a graph illustrating the compressive strength evaluation ofthe bio-based epoxy resin modified by different materials according tosome embodiments of the present disclosure;

FIG. 4 is a graph illustrating the particle morphology of graphite andsilicon dioxide according to some embodiments of the present disclosure;

FIG. 5 is a graph illustrating a variation law of compressive strengthof the bio-based epoxy resin with graphite concentration according tosome embodiments of the present disclosure;

FIG. 6 is a graph illustrating the structural form of the bio-basedepoxy resin that prepared by graphite particles with differentconcentration according to some embodiments of the present disclosure;

FIG. 7 is a schematic diagram illustrating different particle sizes ofthe proppant according to some embodiments of the present disclosure;

FIG. 8 is a graph illustrating the particle size distribution of theproppant that is prepared by the emulsifier with differentconcentrations according to some embodiments of the present disclosure;

FIG. 9 is a result graph illustrating proppant migration andsedimentation experiments according to some embodiments of the presentdisclosure;

FIG. 10 is a graph illustrating the relationship between fracture lengthand deposition height under fracturing fluid with different viscositiesin the proppant migration and sedimentation experiments according tosome embodiments of the present disclosure;

FIG. 11 is a schematic diagram illustrating experimental samples ofcuring phase change proppant, ceramsite and quartz sand according tosome embodiments of the present disclosure;

FIG. 12 is a graph illustrating the fracture permeability of differentproppants at different closure pressures according to some embodimentsof the present disclosure;

FIG. 13 is a graph illustrating the fracture permeability of curingphase change proppant at different temperatures according to someembodiments of the present disclosure; and

FIG. 14 is a graph illustrating the particle morphology of ceramsite,quartz sand and curing phase change proppant according to someembodiments of the present disclosure.

DETAILED DESCRIPTION

To make the technical solutions and advantages of the present disclosureclearer, the technical solutions of the present disclosure will beclearly and completely described below with reference to specificembodiments and accompanying drawings. Obviously, the describedembodiments are only some of the embodiments of the present disclosure,but not all of the embodiments. Based on the embodiments in the presentdisclosure, all other embodiments obtained by the ordinary skilled inthe art without creative efforts shall fall within the protection scopeof the present disclosure.

The embodiments of the present disclosure provide a method for preparinga self-phase change proppant based on an emulsified and toughenedbio-based epoxy resin. The method includes the following steps: S1,preparing the bio-based epoxy resin; S2, performing the tougheningmodification on the bio-based epoxy resin by using the graphiteparticles; S3, using the SiO₂ particles as the emulsifier to emulsifythe bio-based epoxy resin after the toughening modification to preparethe self-phase change proppant that is laid in sections.

In some embodiment, the method for preparing the bio-based epoxy resinin Si may include the following steps.

S101, using dimethyl sulfoxide as a solvent, placing eugenol and1-thioglycerol with a molar ratio of 1:1 and a catalyst4-dimethylaminopyridine (DMAP) with a total mass of 2% of the reactionmonomers (e.g., the eugenol and 1-thioglycerol) in the solvent, andheating them at 65° C. for 4 h to obtain a first product.

S102, reacting the first product and epichlorohydrin in an ethanolsolution with a NaOH mass concentration of 10% at 85° C. for 6 h toobtain a second product.

S103, using a saturated sodium bicarbonate solution to purify the secondproduct to obtain the target bio-based epoxy resin.

The synthetic reaction formula of bio-based epoxy resin is shown in FIG.1 .

In some embodiment, the chemical formula of the bio-based epoxy resinis:

In some embodiments, the molar ratio of eugenol and 1-thioglycerol mayrange from 2:1 to 1:2. In some embodiments, the mass percentage ofcatalyst 4-dimethylaminopyridine (DMAP) may be 1% to 5% of the totalmass of the reaction monomers. In some embodiments, using dimethylsulfoxide as a solvent, placing eugenol and 1-thioglycerol with a molarratio of 1:1 and a catalyst 4-dimethylaminopyridine (DMAP) with a totalmass of 2% of the reaction monomers in the solvent, and heating them at60˜80° C. for 4 h to obtain a pale yellow product. In some embodiments,reacting the pale yellow product with epichlorohydrin in an ethanolsolution with a NaOH mass concentration of 5-15%. In some embodiments,the reaction temperature of the pale yellow reaction product andepichlorohydrin in an ethanol solution with a NaOH mass concentration of10% may be 80˜100° C. For details, refer to embodiment 1.

The synthesis method uses low reaction temperature and short reactiontime, and can effectively prepare bio-based epoxy resins using mildreaction conditions.

In some embodiment, a viscosity and a density of the bio-based epoxyresin at 20° C. are 450˜4700 mPa·s and 1.1˜1.3 g/cm³, respectively. Insome embodiment, the viscosity and the density of the bio-based epoxyresin at 20° C. are 4620 mPa·s and 1.2 g/cm³, respectively.

In some embodiments, after the bio-based epoxy resin is prepared, thetoughening modification may be performed on it. For example, rubberelastomers, core-shell polymers, thermoplastics (TP), thermotropicliquid crystal polymers (TLCP), and/or nanoparticles are added tobio-based epoxy resins for the toughening modification. Nanoparticlesmay include graphite particles, silica particles, and the like. In someembodiments, the bio-based epoxy resin may be toughened by graphiteparticles. Before the bio-based epoxy resin is cured, the bio-basedepoxy resin may be subjected to the toughening modification by aphysical method of adding graphite particles therein. The use ofgraphite particles to perform the toughening modification on thebio-based epoxy resin can make the bio-based epoxy resin have greatercompressive strength and higher toughness. For details, refer toembodiment 2.

In some embodiments, graphite particles with an average particle size of10˜70 μm and a mass concentration of 0.5%˜10% may be added to thebio-based epoxy resin for the toughening modification. In someembodiments, graphite particles with an average particle size of 10˜70μm and a mass concentration of 1%-7% may be added to the bio-based epoxyresin for the toughening modification. In some embodiments, graphiteparticles with an average particle size of 10˜70 μm and a massconcentration of 1%˜5% may be added to the bio-based epoxy resin for thetoughening modification. As shown in FIG. 5 , when the massconcentration of graphite particles is lower than 5%, the compressivestrength of the bio-based epoxy resin increases with the increase of themass concentration of graphite particles. The mass concentration ofgraphite particles increased from 0% to 5%, and the compressive strengthof the bio-based epoxy resin increased from 50.8 MPa to 74.0 MPa.However, as the mass concentration of graphite particles continues toincrease, the compressive strength of the bio-based epoxy resinsdecreases. When the mass concentration of graphite particles increasedto 7%, the compressive strength of the bio-based epoxy resin decreasesto 70.3 MPa. In some embodiments, the mass concentration of graphiteparticles used may be lower than 5%, for example, 2%, 3%, 4%, 5%, etc.

In some embodiments, graphite particles with an average particle size of20˜60 μm, 30˜50 μm etc., may also be added to the bio-based epoxy resinfor the toughening modification. In some embodiments, graphite particleswith an average particle size of 30 μm, 40 μm, 50 μm etc., may also beadded to the bio-based epoxy resin for the toughening modification.

In some embodiments, graphite particles with an average particle size of40 μm and a mass concentration of 5% may be added to the bio-based epoxyresin for the toughening modification. In some embodiments, graphiteparticles with an average particle size of 40 μm and a massconcentration of 4% may be added to the bio-based epoxy resin for thetoughening modification. In some embodiments, graphite particles with anaverage particle size of 40 μm and a mass concentration of 3% may beadded to the bio-based epoxy resin for the toughening modification.

The distribution of graphite particles in bio-based epoxy resin mayaffect the compressive strength of the bio-based epoxy resin. Therefore,by selecting graphite particles with appropriate mass concentration toperform the toughening modification on the bio-based epoxy resin, thebio-based epoxy resin with the strong compressive strength can beprepared. For details on the influence of graphite particles on thecompressive strength of the bio-based epoxy resin, refer to embodiment2, FIG. 5 and related descriptions.

After obtaining the bio-based epoxy resin on which the tougheningmodification is performed, an emulsifier (e.g., SiO₂ particles) may beused to emulsify the bio-based epoxy resin to obtain a self-phase changeproppant. The self-phase change proppant refer to proppant that maychange from a liquid-phase proppant to a solid-phase proppant.

In some embodiments, SiO₂ particles with a particle size of 10˜100 nmmay be selected as the emulsifiers. In some embodiments, SiO₂ particleswith a particle size of 10 nm, 30 nm, 50 nm, 70 nm, 90 nm may beselected as the emulsifiers. The preparation of self-phase changeproppant is carried out by Pickering emulsification technique. SiO₂particles with a particle size of 50 nm is moderate, as an emulsifier,it may effectively prepare a bio-based epoxy resin proppant containingproppant particles with different particle sizes, which can meet therequirements of proppant mesh numbers for propping up the near-wellfractures, main fractures and branch fractures at the same time, andmeet the needs of being laid in different sections.

In some embodiments, a proportion of different mesh numbers in theself-phase change proppant is adjusted by changing a concentration ofthe emulsifier during emulsification. The average particle size of theself-phase change proppant decreases with the increase of the emulsifierconcentration. When the emulsifier concentration is selected from 0.3%to 1%, the different mesh proportions in the self-phase change proppantare different. When the emulsifier concentration is 0.3%, the self-phasechange proppant with the largest proportion is the 12-mesh self-phasechange proppant. When the emulsifier concentration rises to 0.7%, the20-mesh self-phase change proppant occupies the largest proportion. Whenthe emulsifier concentration is 1%, the largest proportion becomes theself-phase change proppant with a particle size of 80 mesh. Theself-phase change proppant with different mesh numbers may be obtainedby selecting the emulsifier concentration according to the siterequirements. By changing the emulsifier concentration, the obtainedproppant may be suitable for different reservoir environments, and theapplication range is expanded. For example, the large-size proppant(e.g., in 12 mesh) may sediment rapidly in the near-well reservoir toprop up the near-well fractures, the 20-mesh proppant may sediment inthe main fractures to prop up the main fractures, and the small-sizeproppant (e.g., in 80 mesh) does not sediment in the near-well fracturesand the main fractures, and can effectively prop up the distal fracturesand branch fractures, thereby meeting the needs of being laid indifferent sections.

In some embodiments, the self-phase change proppant may be prepared byemulsification of bio-based epoxy resins with SiO₂ particles at aconcentration of 0.7% as an emulsifier.

The self-phase change proppant prepared under the above condition issuitable, for example, for use in deep wells with high closurepressures. The self-phase change proppant has sufficient compressivestrength and resistance to wear, and the relative density of theself-phase change proppant particles is low. Therefore, the self-phasechange proppant is suitable for remote fractures and branch fractureswhile ensuring that the proppant can withstand the high pressure andfriction during injection. In addition, the rapid sedimentation of thelarge particle proppant can also prevent the self-phase change proppantfrom clogging tiny fractures.

In some embodiments, the self-phase change proppant may be prepared byemulsification of bio-based epoxy resins with SiO₂ particles at aconcentration of 1% as an emulsifier.

The self-phase change proppant prepared under the above condition issuitable for use, for example, in vertical fractures in deep wells. Theproppant particles with smaller mesh numbers sediment faster and areunevenly laid longitudinally in the vertical fractures, which may affectthe effect of propping up. Therefore, when the fracture length is long,the proppant particles obtained by increasing the emulsifierconcentration have large mesh numbers and a slow sedimentation speed,which is favorable for being evenly laid in the longitudinal directionof the vertical fracture. At the same time, the self-phase changeproppant prepared under the above condition has better sedimentation andmigration ability in longer fractures, and can be evenly laid on thefractures when the viscosity of the fracturing fluid is low (e.g., 1mPa·s), avoiding adjustment fracturing fluid viscosity, therebyimproving the construction efficiency.

In some embodiments, the effectiveness of the proppant may be evaluatedby evaluating the migration ability of the liquid phase proppant and thefracture permeability of the solid phase proppant. The liquid phasechange proppant has good migration ability and can achieve the effect ofbeing laid in different sections, and the solid phase proppant has goodfracture permeability, compressive strength and temperature resistance.For details, refer to embodiment 4.

Embodiments

The experimental methods in the following embodiments are conventionalmethods unless otherwise specified. The test materials used in thefollowing embodiments were purchased from conventional biochemicalreagent companies unless otherwise specified.

Embodiment 1: the preparation and evaluation of the bio-based epoxyresin.

(1) The preparation of the bio-based epoxy resin.

Using dimethyl sulfoxide as a solvent, placing eugenol and1-thioglycerol with a molar ratio of 1:1 and a catalyst4-dimethylaminopyridine (DMAP) with a total mass of 2% of the reactionmonomers in the solvent, and heating them at 65° C. for 4 h to obtain apale yellow product. Reacting the pale yellow product andepichlorohydrin in an ethanol solution with a NaOH mass concentration of10% at 85° C. for 6 h, then being purified by using a saturated sodiumbicarbonate solution to obtain the target bio-based epoxy resin. Thereaction formula and the morphologies of the products are shown in FIG.1 and FIG. 2 , respectively.

(2) The evaluation of the prepared bio-based epoxy resin.

1. Viscosity and density evaluation of the bio-based epoxy resin.

First, the viscosity of the prepared bio-based epoxy resin was evaluatedusing an Anton Pa rheometer at temperatures of 20° C., 40° C., 60° C.and 80° C. with a shear rate of 30/s. Subsequently, the density isobtained by measuring the mass of the bio-based epoxy resin per unitvolume.

TABLE 1 Density and viscosity changes of the bio-based epoxy resinobtained at different temperatures No. Temperature/° C. Viscosity/mPa ·s Density/g/cm³ 1 20 4620 1.2 2 40 1625 1.2 3 60 686 1.2 4 80 428 1.2

It can be seen from the results in Table 1 that the viscosity of theprepared bio-based epoxy resin at 20° C. is 4620 mPa·s, which is muchlower than that of bisphenol-A epoxy resin (11000 mPa·s). The viscosityof the prepared bio-based epoxy resin is very sensitive to temperaturesand decreases to 686 mPa·s when the temperature rises above 60° C. Thedensity of bio-based epoxy resin remains constant at 1.2 g/cm³, which islower than conventional epoxy resin (1.6 g/cm³) and traditional solidproppant (quartz sand: 2.2˜2.3 g/cm³, ceramsite: 1.7˜1.9 g/cm). Thedecrease in density and viscosity of the bio-based epoxy resin is due tothe existence of branched chains with epoxy groups in the molecularstructure of the bio-based epoxy resin, and the three-dimensionalstructure formed when the molecular chains are stacked. Because of thelow viscosity of the prepared bio-based epoxy resin, an emulsionformulated from it has higher stability; its low density makes the phasechange proppant prepared from it have better migration ability.

2. Determination of epoxy value of the bio-based epoxy resin.

Epoxy value is the amount of epoxy group contained in 100 g of epoxyresin, which is the main index for evaluating the performance of epoxyresin. Using the hydrochloric acid-acetone method, the epoxy value ofthe prepared bio-based epoxy resin is evaluated by measuring the amountof hydrochloric acid reacted with a certain amount of epoxy resin. Thecalculation method of epoxy value is:

$\begin{matrix}{{EPV} = \frac{\left( {V_{0} - V} \right)C}{10W}} & \left( {1‐1} \right)\end{matrix}$where V denotes volume of NaOH solution consumed by a sample, mL; V₀denotes volume of NaOH solution consumed by a blank sample, mL; Cdenotes NaOH solution concentration, mol/L; W denotes sample mass, g.

The epoxy value of the prepared bio-based epoxy resin by calculation isabout 0.53 g/Eq, which has a high epoxy value. Therefore, less curingagent is required in the curing process, the application cost is low,and the curing product has high hardness but poor toughness. Therefore,the toughening modification should be performed on the bio-based epoxyresin to increase its compressive strength before application.

Embodiment 2: the toughening modification of the bio-based epoxy resin.

To overcome the defects of poor toughness and notch sensitivity of thebio-based epoxy resin, and to make the proppant prepared based on theprepared bio-based epoxy resin have better compressive strength, theprepared bio-based epoxy resin is performed the toughening modificationby a physical method of adding organic components or inorganic rigidparticles to the bio-based epoxy resin before curing. By evaluating thephysical properties of the bio-based epoxy resin after curing, theoptimal type and dosage of added particles are selected. Theepoxy-resin-curing agent used in the evaluation experiment is an adductof diethylenetriamine and butyl glycidyl ether with a good curingability in a wet environment. The curing agent has a good curing abilityin a wet environment, so that when its dosage is 25% of the mass of theepoxy resin, the epoxy resin may be fully cured within 40 min at anambient temperature of 60° C., which meets the requirements of proppingup reservoirs after field fracturing.

(1) Selection of the materials for performing the tougheningmodification.

To evaluate the impact of different materials for performing thetoughening modification on the compressive strength of the bio-basedepoxy resin, rubber elastomer particles, rigid graphite particles andsilica particles, with an average particle size of 40 μm and a massconcentration of 3% of the resin, are selected to toughen and modify theprepared bio- based epoxy resin. The optimal material for performing thetoughening modification may be selected by evaluating the compressivestrength of the toughened resin. The results of the evaluationexperiment are shown in FIG. 3 , where (a) is unmodified resin, (b) is3% graphite particles, (c) is 3% rubber elastomer particles, and (d) is3% silica particles.

As shown in FIG. 3 , the compressive strength of the resin afterelasticity, rigid material is added has all been improved, and thecompressive strength of the resin with rubber elastomer particlesincreased to 60.3 MPa from 50.8 MPa, and the compressive strength of theresin with rigid graphite particles and silica particles increased to72.1 MPa and 73.5 MPa, respectively, indicating that the tougheningeffect of rigid particles on the epoxy resin is higher than that oforganic fillers. At the same time, the displacements of the resinstoughened by graphite particles and silica particles under compressionare 0.55 mm and 0.21 mm, respectively, which indicates that the resintoughened by graphite particles has better toughness while thecompressive strength is close to that of the resin toughened by silicaparticles.

FIG. 4 is the particle morphology of graphite and silica particles,wherein (a) is graphite, (b) is silica. The better toughness of theresin toughened by graphite particles is due to the higher sphericity ofgraphite particle morphology compared with silica, which reduces theappearance of particle agglomerates and distributes more uniformly inthe resin, thus resulting in better toughness. When used as a proppant,the resin toughened by graphite particles can also effectively prop upfractures when dealing with higher closing pressure, so graphiteparticles are selected as the material for performing the tougheningmodification on the prepared bio-based epoxy resin.

(2) Optimization of toughening material formulation system.

Through the evaluation experiment, it is found that the graphiteparticles have a good effect on performing the toughening modificationon the resin material, and significantly improve the compressivestrength and toughness of the resin material. To obtain a betterconcentration of the graphite particles, the compressive strength ofresin materials containing different concentrations of graphiteparticles is analyzed, the influence mechanism of graphite concentrationon compressive strength of resin materials is analyzed according to themicroscopic surface morphology of the resin, and the evaluation resultsare shown in FIG. 5 .

As may be seen from the compressive strength of resins containingdifferent concentrations of graphite particles in FIG. 5 , when the massconcentration of graphite particles is lower than 5%, the compressivestrength of the bio-based epoxy resin increases with the increase of themass concentration of graphite particles. The mass concentration ofgraphite particles increased from 0% to 5%, and the compressive strengthof the bio-based epoxy resin increased from 50.8 MPa to 74.0 MPa.However, as the mass concentration of graphite particles continues toincrease, the compressive strength of the bio-based epoxy resinsdecreases. When the mass concentration of graphite particles increasedto 7%, the compressive strength of the bio-based epoxy resin decreasesto 70.3 MPa. According to the structural form of the bio-based epoxyresin that prepared by graphite particles with different concentration,it is known that the reduction in the compressive strength of the resinis due to excessive packing of particles. As shown in FIG. 6 , where (a)and (b) correspond to graphite particle concentrations of 5% and 7%,respectively. It may be seen from FIG. 6 that the low-concentrationgraphite particles are uniformly distributed in the resin. At this time,the graphite particles may restrain the deformation of the resin byrestricting the movement of the molecular chains of the adjacent resinmatrix. When the concentration of graphite particles increases, due tothe poor wettability between graphite particles and epoxy resin and thelimitation of the irregular morphology of graphite particles, it maylead to the appearance of graphite particle agglomerates in the resinand the appearance of small holes without resin filling between theparticles of the agglomerates, resulting in a decrease in the strengthof the resin. Therefore, the concentration of the graphite particlesused should be lower than 5%, and the graphite particles with aconcentration of 3% are selected finally for considering the performanceand use cost.

Embodiment 3: Preparation of the self-phase change proppant.

The self-phase change proppant is prepared by Pickering emulsificationtechnology using SiO₂ particles with a particle size of 50 nm as theemulsifier. When the concentration of emulsifier is 0.5%, 6 kinds ofbio-based epoxy resin proppants with different particle sizes are mainlyformed. As shown in FIG. 7 , the proppant particle sizes are 2.9 mm (7mesh), 1.7 mm (12 mesh), 0.95 mm (20 mesh), 0.6 mm (30 mesh), 0.4 mm (40mesh) and 0.18 mm (80 mesh), which can meet the requirements of proppantmesh for propping up the near-well fractures, main fractures and branchfractures at the same time.

To make the prepared proppant suitable for different reservoirenvironments, the particle size distribution of the proppant preparedwith different emulsifier concentrations is analyzed by changing theconcentration of the emulsifier. The particle size distribution of theproppant prepared with the emulsifier concentration of 0.3%, 0.5%, 0.7%and 1% is evaluated by the proppant mass percentage, and the evaluationresults are shown in FIG. 8 .

According to the evaluation results, the average particle size of theself-phase change proppant decreases with the increase of the emulsifierconcentration. When the emulsifier concentration is 0.3%, the self-phasechange proppant with the largest proportion is the 12-mesh self-phasechange proppant. When the emulsifier concentration rises to 0.7%, the20-mesh self-phase change proppant occupies the largest proportion. Whenthe emulsifier concentration is 1%, the largest proportion becomes theself-phase change proppant with a particle size of 80 mesh. Therefore,in the application process, the self-phase change proppant withdifferent mesh numbers may be obtained by selecting the emulsifierconcentration according to the site requirements.

Embodiment 4: evaluation of the effect of the prepared proppant.

(1) Evaluation of the migration ability of liquid phase proppant.

To study the migration and sedimentation ability of the prepared liquidphase proppant in fractures and the effect of different fracturing fluidviscosities on the migration ability of the prepared liquid phaseproppant, three sets of comparative experiments are designed. Themigration ability of the prepared proppant is evaluated using avisualized sedimentation and migration device, and the specificexperimental scheme is shown in Table 2.

TABLE 2 Design of migration and sedimentation experiments of theproppant No. Displacement/m³/h Sand ratio/% Fracturing fluidviscosity/mPa · s 1 5.4 10 1 2 5.4 10 5 3 5.4 10 25

Evaluation experiment result is shown in FIG. 9 and FIG. 10 . It may beseen from the evaluation experimental results that the liquid phaseproppants prepared in some embodiments of the present disclosure havegood migration ability and may achieve the effect of being laid indifferent sections (e.g., different types of fractures, reservoir,etc.). When the fracturing fluid viscosity is 1 mPa·s and the fracturingfluid displacement is 5.4 m³/s, the 7-mesh and 12-mesh proppants withlarge particle sizes sediment rapidly in the near-well reservoir, andthen the 20/40-mesh proppant sediments to prop up the main fractures,but the 80-mesh small particle size proppant does not sediment,indicating that the proppant may be effectively carried by thelow-viscosity fracturing fluid to achieve the effect of being laid indifferent sections. The small particle size proppant may effectivelyprop up the distal fractures and branch fractures, and the rapidsedimentation of the large particle size proppant may also prevent theblockage of the tiny fractures. When the viscosity of fracturing fluidincreases, the migration ability of liquid phase proppant changessignificantly. When the viscosity of the fracturing fluid is 5 mPa·s,the 7-mesh, 12-mesh and 20/40-mesh proppants may all migrate to the mainfractures and distal fractures, and the sedimentation in the near-wellreservoir is significantly reduced. When the viscosity of the fracturingfluid increased to 25 mPa·s, the liquid phase proppant does not showobvious sedimentation, indicating that increasing the viscosity of thefracturing fluid may significantly increase the migration ability of theliquid phase proppant. It can be obtained from the evaluation experimentthat the proppant placement position may be controlled by controllingthe viscosity of the fracturing fluid according to the length of thefracture in the field, to achieve the optimal propping effect.

(2) Evaluation of the fracture permeability of curing phase changeproppant.

The fracture permeability of 40/70-mesh curing phase change proppant,ceramsite proppant and quartz sand proppant obtained under differentclosing pressures are evaluated and tested by using high temperaturehydraulic fracturing and seepage simulation device, respectively, andthe particle morphologies of different proppants are observed. Theexperimental device used to evaluate the fracture permeability mayperform evaluation experiments at a confining pressure of up to 60 MPa.The stainless steel cores produced are spliced by two semi-cylindricalcores, with rough surface fractures at the splices, with a diameter of2.5×10⁻² m and a length of 1.5×10⁻¹ m. As shown in FIGS. 11 , (a), (b),and (c) are experimental samples of the curing phase change proppant,ceramsite and quartz sand, respectively. The test process is as followssteps.

1, Spreading different types of proppants of the same mass in a singlelayer uniformly on the core to form coated proppant fractures.

2, The core holder holds the core and places it in the high pressureseepage simulation device.

3, testing the changing trend of fracture permeability of differenttypes of proppants under different closing pressures (5 MPa-50 MPa) anddifferent ambient temperatures (40° C., 60° C., 90° C.).

The fracture permeability of three different proppants at differentclosing pressures and the curing phase change proppants at differenttemperatures are shown in FIG. 12 and FIG. 13 , respectively.

From the experimental evaluation results of fracture permeability ofdifferent proppants, it can be seen that the curing resin proppant hashigher fracture permeability than quartz sand and ceramsite, and withthe increase of closing pressure, the reduction of the fracturepermeability of the curing resin proppant is lower. This is due to thebetter sphericity of the curing resin proppant. As shown in FIGS. 14 ,(a), (b), and (c) are the particle morphology of ceramsite, quartz sandand the curing phase change proppant, respectively. With the increase ofclosing pressure, the accumulation mode of quartz sand and ceramsitewith lower sphericity changed due to the pressure. When the closingpressure increases to 35 MPa and 50 MPa, the fracture permeability ofquartz sand and ceramsite decrease by 68% and 57%, respectively, and thelarge decrease of fracture permeability is caused by the proppantbreakage. However, the fracture permeability of the curing resinproppant does not decrease significantly when the closing pressure is 50MPa, and as the ambient temperature increases to 90° C., the performanceof the curing resin proppant remains stable, indicating that the curingresin proppant has good fracture permeability, compressive strength andtemperature resistance, which also reflects that the prepared curingphase change proppant has better fracture conductivity than theconventional solid proppant, and is more suitable for reservoirs in hightemperature and high closure stress.

Finally, it should be noted that the above embodiments are only used toillustrate the technical solutions of the present disclosure, but notlimitation. Although the present disclosure has been described in detailwith reference to the foregoing embodiments, the ordinary skilled in theart should understand that the technical solutions described in theforegoing embodiments can still be modified, or some or all of thetechnical features thereof can be equivalently replaced. However, thesemodifications or replacements do not make the essence of thecorresponding technical solutions deviate from the scope of thetechnical solutions of the embodiments of the present disclosure.

What is claimed is:
 1. A method of preparing a self-phase changeproppant based on an emulsified and toughened bio-based epoxy resin,wherein toughening modification is performed on the bio-based epoxyresin by graphite particles, and then the bio-based epoxy resin afterthe toughening modification is emulsified by SiO₂ (silicon dioxide)particles as an emulsifier to prepare the self-phase change proppant; aproportion of different mesh numbers in the self-phase change proppantis adjusted by changing a concentration of the emulsifier duringemulsification; and the chemical formula of the bio-based epoxy resinis:


2. The method of claim 1, comprising: S1, preparing the bio-based epoxyresin; S2, performing the toughening modification on the bio-based epoxyresin by using the graphite particles; and S3, using the SiO₂ particlesas the emulsifier to emulsify the bio-based epoxy resin after thetoughening modification to prepare the self-phase change proppant thatis laid in different sections.
 3. The method of claim 2, wherein a massconcentration of the graphite particles in step S2 is 1%-7%.
 4. Themethod of claim 2, wherein a mass concentration of the graphiteparticles in step S2 is 1%-5%.
 5. The method of claim 2, wherein a massconcentration of the graphite particles in step S2 is 3%.
 6. The methodof claim 2, wherein an average particle size of the graphite particlesin step S2 is 40 μm.
 7. The method of claim 2, wherein a particle sizeof the SiO₂ particles in step S3 is 50 nm.
 8. The method of claim 1,wherein a preparation process of the bio-based epoxy resin includes:S101, using dimethyl sulfoxide as a solvent, placing eugenol and1-thioglycerol with a molar ratio of 1:1 and a catalyst4-dimethylaminopyridine (DMAP) with a total mass of 2% of the reactionmonomerseugenol and 1-thioglycerol in the solvent, and heating them at65° C. for 4 h to obtain a first product; S102, reacting the firstproduct and epichlorohydrin in an ethanol solution with a NaOH massconcentration of 10% at 85° C. for 6 h to obtain a second product,wherein the reaction formula is:

and S103, using a saturated sodium bicarbonate solution to purify thesecond product to obtain the bio-based epoxy resin.
 9. The method ofclaim 1, wherein a viscosity and a density of the bio-based epoxy resinat 20° C. are 4620 mPa·s and 1.2 g/cm³, respectively.